- What can satellite accretion tell us about the Galilean satellites?
- The circumstances of accretion yield the starting template for the rest of a planetary body's history, both in a compositional and thermal sense.
- Accretionary models help explain surface differences on satellites
- Starting Point: The Solar System Example
- An accretion disk forms, in which T rises (solar power, gravitational collapse) then falls, but at all times T<300oK.
- Gravitational instabilites within the orbital motion of the rotating disk yield protoplanets (a high density disk is required)
- Free iron condenses first, followed by major silicates (so the Moon and Mercury, for instance, can't be explained solely by ss position)
- The most volatile abundant ices never can directly condense at suggested nebula T
- Dust grains (condensates) nucleate at the middle of the disk since particles are subject to ballistic and not P considerations. Little turbulence means gravitational instabilities formed by midplane particle concentration, the nucleus for planetesimals km in radius. (Turbulence means no gravitational instabilities, but sticking may form planetesimals)
- Why it works:
- Observation of systems show a large central body with a regular family of much smaller ones.
- Especially for gas giant systems, a gas accretion disk should be formed, from which planetesimals can accrete.
- Volatiles are more abundant with distance.
- Where it breaks down:
- Not all systems have the density gradient that would be expected for solar nebula-type formation
- Size gradient is not consistent
- We are looking at very different materials when we look at satellite formation.
- For Satellites this means.....
We need, therefore, to answer the question: How do ices behave during satellite accretion?
- Model works to a first order, but...
- We do not have the benefit of a central heat source as large as the sun, so the building blocks are different.
For various P-T conditions, a path of condensation can be traced.
- Assuming cold (T<300oK) accretion models for satellite
formation (either gas-rich or gas-free), Lewis (1972) suggests two extreme ways to examine the condensation of satellites within the cold accretion model:
- Equilibrium accretion--cooling of the accretion disk occurs before accretion is completed so that all chemical species are in equilibrium with each other.
- Fractional accretion--accretion is more rapid than chemical cooling so that condensates, once formed, are removed from the condensation system. That is, those condensates that form first are removed and thus unavailable for bonding with other chemical species.